An optical storage system is composed of an optical disc drive system and an optical storage medium, such as an optical disc. The optical disc drive has a light beam source, a light beam distribution system and a light beam detection system for reading out information from the optical disc and recording information onto the optical disc. The read and write operation functionality of the optical disc drives is traditionally accomplished by using an optical pick-up unit (OPU) which is arranged to locate so that the light beam is angled perpendicular to the optical disc for radiating and detecting purposes. This kind of read and write operation is designated as the perpendicular operation in this present application.
The optical pick-up units of prior art typically have a laser light emitting diode, light detector, optical lens and device e.g. voice coil to position the lens for proper focusing and tracking during read and write operation. The OPU moves radially to access data tracks on the optical disc by using a sliding rail system connected to a motor or the fixed OPU is connected to a geared rotatably positioned optical pipe connected to a motor. When the optical disc rotates around its center by means of a motor and the OPU or a light guide head connected to the OPU moves radially across the optical disc, data tracks of the optical disc are accessed.
The optical pick-up units of prior art typically use a single light beam path toward the optical storage medium. Optical light guides are used as a channel for directing the light beam from the light source to the lens system adjacent to the track of the optical disc and for directing back the reflected light beam from the track of the optical disc to the detection system adjacent to the light source. In a typical embodiment, a polarizing beam splitter (or similar system using e.g. semi-reflecting mirrors) is used to direct the light beam to the lens system.
The traditional CD and DVD technology is considered to be known art and covered extensively by patents and published literature; hence, it is not mentioned here explicitly. Some examples of non-standard solutions for prior art optical pick-up units are described in the following documents: WO 99/00793, U.S. Pat. Nos. 4,581,529, 5,481,515, 5,835,458 and 6,256,283. Recent prior art for non-standard fixed-arm systems includes documents WO 02/059888A2 and WO 02/059887A2.
There are certain limitations related to the OPU systems of prior art. A mass of the movable OPU or the geared rotatably positioned optical light guide head is heavy. Especially a laser source is a weighty and large-size component and the mass of the laser source is centered on the movable OPU or the movable light guide head in prior art systems. The weight of the movable OPU causes together with the pitching motion of the disc problems in defocussing and sensitivity to the track angle error. Many optical storage systems of prior art require astigmatism in the system for error analysis, and this also results in higher component count in the form of used astigmatism elements. All extra components cause weight increase and complexity to the system which extends access times and increases power consumption of the OPU system. The access times are outstandingly long in case of the movable sledge OPU systems.